Oxygen stoichiometry and structural properties of La1-xAxMnO3 ± δ (A = Ca or Sr and 0≤x≤1)

Oxygen non-stoichiometry, structural properties, and phase relations in Ca and Sr substituted LaMnO3+/-delta were studied by means of thermal analysis, X-ray diffraction, and iodometric titration. Hexagonal alpha-SrMnO3-delta and LaMnO3+/-delta perovskite are only partly miscible. Room temperature metastable La1-xSrxMnO3+/-delta solid solutions, prepared by quenching from high temperatures where there is complete solid solubility between cubic beta-SrMnO3-delta and LaMnO3 +/- delta perovskite, have rhombohedral, tetragonal or cubic symmetry dependent on the Sr content. Orthorhombic LaMnO3+delta and CaMnO3-delta form a stable La1-xCaxMnO3 +/- delta solid solution in the whole composition region. The unit cell volume of La1-xSrxMnO3 +/- delta and La1-xCaxMnO3 +/- delta solid solutions is reduced with decreasing La-content mainly due to the correlated change in average oxidation state from Mn3+ to Mn4+. The oxygen stoichiometry changes from cation deficient (oxygen excess) at high La-content to oxygen deficiency at low La-content. The enthalpy of the phase transitions of CaMnO3-delta and the transition from alpha-SrMnO3 (hexagonal) to beta-SrMnO3-delta (perovskite) were obtained by differential thermal analysis. Finally, the physical appearance of first order phase transition between different perovskite-related structures is analysed in terms of the Gibbs phase rule. First order transitions between different structures of a ternary oxide ABO(3-delta) become two-phase regions in a quasi-binary system like ABO(3-)delta-A'BO3-delta.